The present invention relates to a guard circuit for the protection of capacitive loads.
In the operation of electronic systems connected to AC supply networks, not only very short interference voltages in the high-frequency range must be expected, which may be filtered out through low-pass filters of small dimensions with a high limit frequency, but also interference pulses whose half-amplitude pulse duration comes within the range of milliseconds. This holds true particularly when industrial machinery, rail-bound systems, heating installations or the like are operated from the supply network. Since the rising edges of such interference pulses are very steep the risk of interference with an electronic equipment is particularly high when the latter constitutes a capacitive load connected to the network. The DIN Standard VDE 0160/01.86 defines these voltage pulses so that an ultimate purchaser of an electronic device tested in compliance with that Standard may operate the unit from such a network without any further hesitation.
In order to protect the unit from such interference pulses it is possible to connect, for instance, a magnetic stabilizer or regulator or even only an isolating transformer ahead of the device. On the other hand, however, this series connection of such an additional element entails comparatively high costs and on the other hand such devices are voluminous in the event of major demands to performance, which counteracts, however, the miniaturization attempts which are always present in electronics.
From the German technical journal DEZ "Elektrotechink", issue of Jun. 1977, No. 11, pages 14 to 19, examples of circuits for the protection specifically of a push-pull voltage converter have become known. In particular, that publication proposes the connection of a current-limiting resistor ahead of the power rectifier. This measure, however, results in a reduction of the period between link-up with the supply network and the time by which the equipment reaches its full capacity. Moreover, the circuit includes a filter capacitor and switching transistors intended to limit overcurrents. The measures taken in that example, however, are not suitable to block out reasonably pulse-type overvoltages of the kind defined by way of introduction hereto.
From the East German Patent No. DD 150 820, a circuit arrangement has become known for the reduction of the short-circuit current in switching network elements, which includes an LC member. This circuit is not suitable either to block pulse-type overvoltages but rather offers itself only for short-circuit protection of parts of a switching network.
In the publication "Schaltbeispiele mit diskreten Halbleiterbauelementen" (Switching Examples with Discrete Semiconductor Elements) by the Intermetall company, issue 1973/3, pages 158 and 159, a transistor switch for AC loads is described which includes a transistor as the controllable switch element in the cross branch of a bridge circuit made up by diodes in anti-parallel arrangement. This circuit, however, is not suitable to block off pulse-type overvoltages.
In the U.S. Pat. No. 44 14 598, a controlled power supply is described which includes an opto-coupler for electric isolation of a blocking signal. With this system, however, it is not possible either to suppress pulse-type overvoltages.
It has become known from the European Patent No. EP-A-130 254 that it is possible to supply a circuit for a line disconnector with power from the supply network. That publication, however, does not disclose any reference to the blocking of pulse-type overvoltages.